Motor drive system with acceleration-deceleration control

ABSTRACT

A drive system utilizes silicon-controlled rectifiers and a feedback loop for operating a direct current motor from alternating current input power at a predetermined speed regardless of load conditions. Circuitry is provided for controlling the width of the dead band of the system, and a circuit is provided for controlling acceleration and deceleration of the motor, said circuit having a capacitor and switch means responsive to an error signal to charge and discharge said capacitor.

United States Patent 51 3,636,424 Reed Jan. 18, 1972 [541 MOTOR DRIVESYSTEM WITH 3,428,880 2/1969 Muller ..31s/2s7- ACCELERATION-DECELERATION3,431,479 3/1969 .loslyn ..3 18/327 X CONTROL 3,457,485 7/1969 Leonard..318/257 3,293,522 12/1966 Lewis ..3l8/257 X [72] Inventor: John F.Reed, Middleburg Heights, Ohio 3,361,921 1/1968 Montross et a]. .13 l8/34l X 3,439,246 4/1969 Moritz ..3i8/257 [m Ass'gnec' gr z fgg 'i'3.579.065 5/1071 hmknilis .3 18/34] x [22] Filed: June23, 1969 PrimaryExaminer'l. E. Lynch Assistant ExaminerRobert J. Hickey [21 1 ApplAttorney-David S. Urey, Alan C. Rose and Alfred B. Levine [52] U.S.CI.3l8/259,3l8/34l [57] ABSTRACT [51] Int. Cl. ..H02p 5/16 A drive 5 ystemutilizes silicon-controlled rectifiers and a feed- [58] Field :11 15 /23?- back loop for operating a direct current motor from alternab 398 ingcurrent input power at a predetermined speed regardless of loadconditions. Circuitry is provided for controlling the width of the deadband of the system, and a circuit is provided [56] References Cited forcontrolling acceleration and deceleration of the motor, UNlTED STATESPATENTS said circuit having a capacitor and switch means responsive to.an error signal to charge and discharge said capacitor. 2,778,9821/1957 Loeffler ..318/257 3,249,838 5/ 1966 Mierendorf ..3 18/257 8Claims, 6 Drawing Figures v FED 7 TACH f AUTO- SPEED 45 CONTROL. 55INPUT m me/501v 50 25 f anew/7-- MANUAL 90 T0 mess-M4455 7'0 4? POWERINP T ACCH'DEC' 7b 44 TRANSFORM R C/ECU/T 4 f 46 /24 DEAD 54ND C/ECU/ Tl 201 /20 3d 7' @475 CONTEOL com/20L 1 2 FIE/N6 16652 W ace FEOM 264 i-GATE 2 CONTEOL CONTROL *1 i @5 1? TP/GGEE 3 ace O PATENTED JANI8I972313351 4 SHEET 2 OF 4 aura.

CONTROL 5a //4 Fig.4

I INVENTOR. JOHN F. EEED Q Q MW,

A TTOEAJEYS.

PATENTED JAN18I972 424 sum 3 or 4 INVENTOR JOHN F; 2550 ATTOENEYE MOTORDRIVE SYSTEM WITH ACCELERATION- DECELERATION CONTROL FIELD OF THEINVENTION This invention relates to direct current motor'drive systems,operated from alternating current input power, which maintain apredetermined motor speed regardless of the load conditions imposedthereon, and to a drive for a machine tool slidable member, which drivewill maintain a predetermined speed of movement of a slide.

SUMMARY or TFIE INVENTION A reversible direct current (DC) motor drivesa load. The motor is energized by three-phase alternating current inputpower through the control system of the invention.

The DC motor also drives a tachometer that providesa voltage signal ofone polarity or another depending on the direction of rotation of themotor, which voltage signal accurately reflects the speed of rotation ofthe motor and hence the speed of drive of the load in one direction. Theoutput from the tachometer is compared with a reference signal thatindicates a desired speed of the motor. If the reference signal and thesignal from the tachometer are not equal in amplitude,silicon-controlled rectifiers (SCRs) that rectify the input powersupplied to the DC motor are caused to become conductive forgreater orlesser portions of each half-cycle to increase or decrease the speed ofthe motor and thus vary the output signal from the tachometer until itand the reference speed signal are equal. Circuitry is provided forcontrolling the width of the dead band of the system. By dead band ismeant the range of values over which the difference between thetachometer output signal and the reference speed input signal can varywithout affecting the output of the control system. In addition, thereis provided a circuit for controlling acceleration and deceleration ofthe motor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustrationof a machine tool with which the control system of the invention may beused; FIG. 2 is-a block diagram of a control system embodying theinvention;

FIG. 3 is a circuit diagram of a comparison circuit and anacceleration-deceleration circuit shown in block form in FIG.

FIG. 4 is a circuitdiagram of a circuit for controlling null balance anddead band shown in block form in FIG. 2 as a dead band circuit; 7

FIG. 5 is a circuit diagram of a firing-gate generator and triggercircuit shown in block form in FIG. 2; and 7 FIG. 6 is a circuitdiagram-of controlled-rectifier circuits shown in block form in FIG. 2.I

DESCRIPTION OF A PREFERRED EMBODIMENT Although the present invention isnot limited to any particular application or applications, it isparticularly useful when applied to the drive mechanism for a machinetool.

Numerical-controlled machine tools having multiple slides are becomingincreasingly popular and economically attractive. In such numericallycontrolled machine tools, it is a virtual necessity that the drives onthe various slides be independently controllable and capable ofoperating at different speeds under the control of a predeterminedprogram. Also, those drive speeds must be controlled with extremeaccuracy to maintain the dimensions of a machined part automaticallywithin the tolerances required in many present-day applications.

The present invention is particularly applicable to such machines as itprovides a drive which will actuate a slide as required, includingholding a slide stationary accurately maintaining a selected slide speedunder all conditions including certain types of machining operations inwhich the movement of a cutting tool tends to increase the speed ofmovement of a workpiece past the cutting tool varying the slide speedswithout stopping movement of the slide, etc. The first-mentioned featureis particularly important when one considers a climb cutting millingoperation, such as is shown diagrammatically in FIG. 1. In thiswell-known type of milling operation, the cutting tool is cuttingdownwardly into a workpiece. This obviously tends to increase the speedof movement of the workpiece past the cutting tool. This is undesirablebecause it tends to degrade the quality of cut taken by the tool in theworkpiece.

A typical machine tool to which the control system of the invention maybe applied is shown diagrammatically in FIG. 1. A workpiece 10 ismounted on a table or slide 12 for movement past a cutting tool 14. Theslide 12 is mounted on ways 16 and is driven by a screw 18. The screw 18is rotated by a reversible DC motor 20 through a conventional gearbox22. An electric tachometer 24 is also driven by the motor 20 andprovides a DC output electric signal that is proportional in amplitudeto the speed of the motor and of a polarity determined by the directionof rotation of the motor. The output signal from the tachometer 24 isutilized as a feedback signal in the speed control system of theinvention. Although only one slide 12 is shown, it is understood thatother slides may be provided, each with its own motor and screw topermit the usual movement along various axes.

It is apparent from FIG. 1 that if the slide 12 is being driven by thescrew 18 to the right and the cutting tool 14 is rotating in acounterclockwise direction (as shown by the arrows), the action of thecutting tool on the workpiece 10 will tend to increase the speed ofmovement of the slide 12. This action will tend to increase the speed ofthe motor 20 and hence increase the amplitude of the output signal fromthe tachometer 24. The drive system of the invention responds to theincrease in the tachometer signal amplitude and operates to maintain thespeed of the motor 20 at a substantially constant predetermined value.

FIG. 2 is a block diagram of a drive system embodying the invention. Itis to be understood that in the following description, the termsignal(s)" means electric signal(s)." The drive motor 20 is powered froma secondary winding of a three-phase power input transformer 26. Currentis supplied to the motor 20 from the transformer 26 through threesiliconcontrolled rectifier (SCR) control circuits 28, 30, 32, and hencethe speed at which the motor 20 will rotate and its direction, arecontrolled by trigger circuits 34, 36, 38, respectively. v

Each of the triggers 34, 36, 38 receives three pairs of input signals.Firing gate controls 40, 42, 44, respectively, provide one pair of theinput signals to the triggers 34, 36, 38. A second pair of input signalsis provided to all of the triggers from a dead band circuit 46, and athird pair of input signals is provided to all of the triggers from acomparison circuit 48.

Each firing gate control 40, 42, 44 determines the exact period andduration of time during which a particular SCR in the SCR controlcircuits 28, 30, 32 may have a trigger pulse applied to it from thetriggers 34, 36, 38. The firing gate controls 40, 42, 44 provideenabling signals that permit the triggers 34, 36, 38 to apply triggerpulses to the various SCRs in the SCR controls only during that periodof time when the anodes of those particular SCRs to be triggered arepositive. To accomplish this purpose, power is supplied to the firinggate controls 40, 42, 44 on leads 50, 52, 54, respectively, from thesecondary winding of the three-phase power input transformer 26.

The dead band circuit 46 provides a pair of signals to the triggers 34,36, 38 that control the width of the dead band of the system. That is,the signals from the dead band circuit 46 control the range of valuesover which input signals to the triginput signals and a predeterminedthat are propertional to the difference in amplitude between the twoinput signals to the circuit. One of the input signals is provided fromthe tachometer 24 which, as previously pointed out, is driven by themotor 20 and provides an output signal that is proportional to the motorspeed and of a polarity determined by the direction of rotation of themotor. The other input signal to the comparison circuit comes eitherfrom an automatic speed control 56 or from a manual speed control 58.The signals from the speed controls 56, 58 serve as reference signals tocontrol the speed and direction of rotation of the motor 20.

The difference signal in the comparison circuit 48 between the signalsfrom either the automatic speed control 56 or the manual speed control58 and the tachometer 24 is modified by a timedacceleration-deeeleration circuit 60. The circuit 60 serves to preventany fast change in speed of the drive motor 20. In other words, thecircuit 60 acts to adjust the electrical time constant of the controlcircuit to match the mechanical time constant of the drive system. Inaddition, it prevents following circuitry from going into saturation,and allows the motor to change to a different desired speed undercontrol of the speed control system.

FIG. 3 is a schematic diagram (partially in block form) of thecomparison circuit 48 and its input circuitry, and of the timedacceleration-deceleration circuit 60. The comparison circuit 48 includesan operational amplifier 62, which is provided with the usual parallelcombination of a feedback resistor 64 and a capacitor 66. input to theamplifier 62 is from the output of the tachometer 24 through a variableresistor 68, and from either the automatic speed control 56 or themanual speed control 58 through a resistor 70. The polarities of thesignals provided from the tachometer 24 and from either of the speedcontrol circuits 56, 58 are opposite. They are added algebraically andthe resultant representing the difference in amplitude between the twosignals is amplified by the amplifier 62. The variable resistor 68provides a calibration for initially balancing the tachometeroutputsignal and the reference signal when the motor is rotating at thecorrect speed.

The automatic speed control 56 may take any one of a number of forms.For example, it may be the output circuitry of a computer that providesa signal that is proportional to a desired motor drive speed, as mightbe the case in a numerical control application. The manual speed control58 might take the form of a rotary switch, each position of whichprovides a different fixed reference voltage to the comparison circuitto indicate different desired speeds.

The automatic speed control 56 is connected to the resistor 70 through anormally closed section 72a of a relay 72. The relay 70 is connected tobe energized from a source of positive voltage (not shown) through anormally open, manually actuatable switch 74. When the switch 74 is openas shown, the signal from the automatic speed control 56 is connectedinto the summing amplifier 62.

The relay 72 has a second, normally open section 72b, which serves toconnect the manual speed control 58 to the input of the amplifier 62.When the manually actuable switch 74 is closed, the contact 72a opensand the contact 72b closes. Thus, the reference signal source istransferred from the automatic speed control 56 to the manual speedcontrol 58.

The output of the operational amplifier 62 is connected through a fixedresistor 82 and a series-connected variable resistor 84 to an input of acomplementary output amplifier 86. The variable resistor '84 serves as asystem gain control. The amplifier 86 provides linearly complementaryoutput signals that vary about a predetermined level such, for example,as volts. That is, as one output signal goes more positive, the otheroutput signal goes more negative by an equal amount and vice versa.These complementary signals are provided on output leads 88, 90.

The timed acceleration-deceleration circuit 60 is connected by means ofa lead 92 to a juncture between the resistors 82, 84. The circuit 60comprises an NPN-transistor 94 and a PNP- transistor 96, which serve asswitches. The emitters of the emitters of the transistors transistors94, 96 are connected together and to the lead 92. The collector of thetransistor 94 is connected through a resistor 98 to a positive potentialsource (not shown), and the collector of the transistor 96 is connectedthrough a resistor 100 to a negative potential source (not shown). Thebases of the transistors 94, 96 are respectively connected throughresistors 102, 104 to one side of a capacitor 106, the other side ofwhich is grounded.

In operation, if the voltage applied to the emitter of the transistor 96through the lead 92 exceeds the base-to-emitter standoff voltage of thattransistor, then the transistor 96 will start to conduct. This causesthe capacitor 106 to charge through the emitter-base circuit of thetransistor 96 until its charge approximately equals the emitter voltageof the conducting transistor 96. The charging rate of the capacitor 106determines the rate at which current is supplied to the input of thecomplementary output amplifier 86. When the input voltage through thelead 92 changes so that it exceeds the base-toemitter standoff voltageof the transistor 94, the same action occurs, but conduction is throughthe transistor 94 to' discharge the capacitor 106 until its chargeapproximates the emitter voltage of the transistor 94. In other words,if the voltage on the emitters of the transistors 94, 96 goes suddenlyin a positive direction, the transistor 96 will conduct to charge thecapacitor 106 positively. Conversely, if the voltage on the goessuddenly in a negative direction, the transistor 94 will conduct todischarge the capacitor 106. Thus, a timed acceleration or decelerationfunction is obtained by charging and discharging the capacitor 106.

The particular advantage of using the transistors 94, 96 in theconfiguration shown is that the charging and discharging rates of commoncapacitor 106 taken advantage of the current gain (beta) of thetransistors and allows a much smaller capacitor to be used than if apassive resistance-capacitance network were used.

The dead band circuit 46 shown in FIG. 4 performs two functions. First,it serves to set the dead band of the control system, within which itwill not respond to variations in output signals from the complementaryamplifier 86. Second, it provides a signal to the triggers 34, 36, 38 toadjust each of the triggers so that each trigger responds in a similarmanner to both positive and negative half-cycles of the input voltage.The circuit 46 comprises a diode 108, whose anode is connected throughaswitch 1 10 to a positive potential source (not shown). The cathode ofthe diode 108 is connected through a potentiometer 112 to the cathode ofa Zener diode 114; the anode of the Zener diode 1 14 is grounded. Twofixed resistors 116, 118 are connected in series between a movable armof the potentiometer 112 and the cathode of the Zener diode 114. Anoutput signal is obtained on a lead 120 connected to a juncture betweenfixed resistors 116, 118. A potentiometer 122 is similarly connectedbetween the movable arm of the potentiometer 112 and the cathode of theZener diode 114. An output signal is also obtained on a lead 124connected to a movable arm of the potentiometer 122. The leads 120, 124serve as input leads to the triggers 34, 36, 38.

In operation, the switch serves as an on-off switch for the controlsystem. If the switch 110 is open, no output signals will be providedfrom the triggers 34, 36, 38 to the SCR controls 28, 30, 32 and themotor 20 will not be energized. When the switch 110 is closed as shown,certain positive voltages will be applied through the leads 120, 124 tothe triggers 34, 36, 38. The voltages so applied are basicallycontrolled by the setting of the movable arm of the potentiometer 112.As will later become apparent in connection with the description of thetriggers, the setting of the movable arm of the potentiometer 112determines the dead band of the control system. The purpose of thepotentiometer 122 is to permit the signals on the leads 120, 124 to beequal in amplitude. Thus the potentiometer 122 serves as a balanceadjustment for the portions of each trigger circuit that respond topositive-going and negative-going half-cycles of input voltage.

. PK]. 5 illustrates the firing gate control 40 and the trigger 34 inschematic form. Inasmuch as the firing-gate controls 40, 42, 44 areidentical and the triggers 34, 36, 38 are identical, only thefiring-gate control 40 and the trigger 34 will be described in detail.

The firing-gate control 40 comprises a transformer 130 having a primarywinding 130? and a secondary winding 1305. One end of the primarywinding 130? is connected to the lead 50 from the three-phase powerinput transformer 26 (FIG. 1), andthe other end of the primary windingis connected to a lead 132.'The secondary of the power input transformer26 is connected in the form of a Y-configuration (as best seen in F IG.6), and the lead 132 is connected to the center of the Y. The lead 132is common to all three of the firing-gate controls 40, 42, 44.

A center tap on the secondary winding 130$ of the transformer isconnected to a negative potential source (not shown). Opposite ends ofthe secondary winding 130S are respectively connected through resistors134, 136 to the cathodes of diodes 138, 140. The anodes of the diodes138, 140 are connected together and to the center tap .on the secondarywinding 1308 of the transformer 130. The secondary winding 130$ of thetransformer 130 provides positive voltages to the bases of twotransistors in the trigger circuit 34. The purpose of the diodes 138,140 is to prevent breakdown of these transistors when the voltagesacross the secondary winding 130$ reverse in polarity.

The trigger 34 is divided into two sections shown as upper and lowermirror images in FIG. 5. One section responds to one half-cycle of eachalternating current input cycle and the other section responds to theother half cycle. Only one section will be describedin detail. The samereference numerals are applied to like components of both sections butwith those of the upper section being shown followed by a prime suffix.

The lower section of the trigger 34 comprises a PNP- charge-controltransistor 142, an NPN-gate transistor 144, and a unijunction transistor146. The upper section comprises similar transistors 142, 144 and 146'.I The input connections to the transistors 142, 142' are different, butotherwise the circuitry of the two sections is identical. The base ofthe transistor 142 is connected to the lead 90 from the complementaryamplifier 86 in the comparison circuit 48, while the base of thetransistor 142 is connected to the lead 88 from thearnplifier 86. Theemitter of the transistor 142 is connected to the lead 120 from the deadband circuit 46, while the emitter of the transistor 142 is connected tothe lead 124 from the dead band circuit. With those noted differences,the upper and lower sections of the trigger are identical and only thelower section shown in FIG. 5 will be described.

The collector of the transistor 142 is connected through a resistor 148to the collector of the transistor 144. The emitter of the transistor144 is connected to a negative potential source (not shown) by means ofa lead 150. A charge control capacitor 152 is connected between thecollector of the transistor 142 and the negative lead 150. When thecharge control transistor 142 is conductive, the charge controlcapacitor 152 will charge; when the gate transistor 144 is conductive,the charge control capacitor is shorted by the transistor 144.

The collector of the charge control transistor 142 and one side of thecharge control capacitor 152 are connected to the emitter of theunijunction transistor 146. One base of the transistor 146 is connectedthrough a resistor 154 to a positive potential source (not shown) andthe other base of the transistor 146 is connected through a resistor 156to the negative lead 150. The base of the transistor 146 that isconnected to the resistor 156 is also connected through a capacitor 158and a series-connected resistor 160 to the base of an NPN-buffer-amplifier transistor 162. The base of the transistor 162 is alsogrounded through a resistor 164. The collector of the transistor 162 isconnected to ground through a primary winding 166? of a pulsetransformer 166. The emitter of the transistor 162 is connected to thenegative lead 150 through a parallel combination of a capacitor 168 anda resistor 170.

The pulse transformer 166 has a pair of secondary windings 166S, whichare connected in parallel. The primary and secondary windings of thetransformer 166 are so arranged that negative and positive outputsignals are provided on leads 172, 174, respectively, from the secondarywinding 166$ when the transistor 162 conducts.

As an aid to understanding the operation of the circuitry thus fardescribed, assume first that the drive motor 20 is in a desireddirection but is not running at the desired speed. Therefore, there willbe a signal of a given polarity and amplitude at the input to theoperational amplifier 62. This will cause an unbalance between theoutput signals of the com plementary amplifier 86. For example, theoutput signal on the lead 88 might be at +12 volts, while the signal onthe output lead 90 might be at +8 volts. Further assume that the deadband circuit 46 has been so adjusted that the output signals on theleads 120, 124 are both approximately +l0 volts. This means that thebase of the transistor 142' in the trigger circuit will be more positivethan the emitter of that transistor, and so the transistor 142' will benonconductive and the charge control capacitor 152 will not charge. Onthe other hand, the emitter of the transistor 142 in the other sectionof the trigger circuit will be positive with respect to the base of thetransistor, which will permit the charge control capacitor 152 I tocharge at a predetermined rate. When the capacitor 152 has charged tothe emitted peak voltage point of the unijunction transistor 146, thetransistor 146 conducts and the capacitor 152 discharges through thetransistor 146 and the resistor 156. This provides a positive pulsethrough the capacitor 158 and the resistor 160 to the base of the bufferamplifier transistor 162, which causes that transistor to conduct. Theresulting output pulse from the transistor 162 is transmitted throughthe pulse transformer 166 to the output leads 172, 174.

If now the motor is caused to reverse direction but is not running at adesired speed, the input signal to the operational amplifier 62 in thecomparison circuit 48 will be of opposite polarity to that previouslydescribed. The signal on the output lead 88 will be less positive thanl0 volts and the signal on the output lead will be more positive than l0volts. This will cause the charge control transistor 142' in the trigger34 to become conductive and the transistor 142 to become nonconductive.Thus, discharge of the charge control capacitor 152 through theunijunction transistor 146' will cause output pulses to appear on theleads 172', 174'. As will be later explained, output pulses on the leads172, 174 cause the motor 20 to rotate in one direction, while outputpulses on the leads 172, 174' cause the motor to rotate in a reversedirection.

The purpose of the gate transistors 144, 144 is to permit theirrespective charge control capacitors 152, 152 to discharge alternatelyeach half cycle. That is, when the upper end of the transformersecondary winding goes positive each alternate half-cycle, thetransistor 144 becomes conductive and effectively short circuits thecharge control capacitor 152 to discharge it. A similar action occursduring alternate half-cycles wherein the gate capacitor 144 becomesconductive to discharge the charge control capacitor 152. Thus, neitherof the charge control capacitors 150, 152 can charge for more than onehalf-cycle of the alternating current input voltage to the SCR controlassociated with that particular trigger.

As previously pointed out, the signals from the comparison circuit onthe leads 88, 90 vary in amplitude about a fixed positive voltage levelin accordance with the amplitude and polarity of the error signal at theinput of the complementary amplifier 86. If the error signal is of onepolarity, the signal on the lead 88 will be more positive than that onthe lead 90. If the error signal is of the other polarity, the signal onthe lead 90 will be more positive than that on the lead 88.

Which of the signals is most negative controls which of the chargecontrol transistors 142, 142' becomes conductive. The

degree to which either transistor 142, 142 conducts depends on theamount by which its emitter potential (on leads 120 or 124 from the deadband circuit 46) exceeds its base potential (on leads 88 or 90 from thecomplementary amplifier 86). The degrees of conduction of thetransistors 142, 142' respectively control the harging time constants ofthe charge control capacitors 152, 152'. This, of course, determines thetime in a half-cycle when the charge across one of the capacitors 152,152 will'equal the emitter peak voltage level of its associatedunijunction transistor 146, 146' and cause an output pulse to appear onthe leads 172, 174 or 172', 174.

'ilt is pointed out that the control system does not permit the motor torun at an absolutely constant speed. There is always some slightjockeying between the actual speed of the motor and the desired speed ofthe motor. This is caused by the fact that there must be some differencein .the output signals on the leads 88, 90 from the complementaryamplifier 86 in the comparison circuit 48 to cause any rotation of themotor. If both signals from the comparison circuit 48 are equal to bothsignals from the dead band circuit 46, neither of the charge controltransistors 142, 142' will become conductive and there will be no powersupplied to the motor. Nevertheless, it has been found in practice thata desired speed of rotation of the motor 20 can be controlled undervarious load conditions to within 1 percent of the desired value.

FIG. 6 illustrates the three-phase power input transformer 26 and theSCR controls 28, 30, 32. As shown, the power input transformer has threeprimary windings 26P1-2 6P3 connected in a delta configuration. It alsohas three secondary windings 26Sl-S3 connected in a Y-configuration. Acommon point of the secondary windings is connected to the lead 132previously mentioned in connection with FIG. 5. Each of the secondarywindings 26S1-S3 is tapped to provide the signals on the conductors 50,52,54 to the firing gate controls 40, 42, 44, respectively. Each of thetransformer secondary windings is also provided with a seriescombination of a capacitor and a resistor for transient suppression. Thewinding 26S1 has connected thereacross a resistor 180 and a capacitor182; similarly connected across the winding 2652 are a resistor 184 anda capacitor 186; and a resistor 188 and a capacitor 190 are connected inthe same fashion across the winding 2653.

The SCR controls 28, 30, 32 are respectively connected to the outer endsof the secondary transformer windings 26S], 26S2, 26S3. Inasmuch as theSCR controls 28, 30, 32 are identical to each other in construction, anddiffer only in the source from which they obtain their input signals,only the SCR control 28 will be described in detail. 1

The control 28 comprises a pair of SCRs 190, 192. The anode of the SCR190 and the cathode of the SCR 192 are connected to the outer end of thesecondary winding 26Sl. The cathode of the SCR 190 and the anode of theSCR 192 are connected through a lead 194 to one side of the DCreversible motor 20. The other side of the motor 20 is connected to thecenter point of the Y of the transformer secondary through a lead 196. Athyrector 198 is connected in parallel with the SCRs 190, 192. Thethyrector 198 comprises backto-back, selenium rectifiers which serve toclip any transients that might cause the SCR's 190, 192 to fire falsely.The SCR 190 has a gate electrode which is connected to the lead 174'from the trigger 34. The cathode of the SCR 190 is connected to thelead172' from the same trigger. The SCR 192 has a gate electrode that isconnected to the lead 174 from the trigger 34, and the cathode of theSCR 192 is connected to the lead 172 from that same trigger. The SCRcontrols 30, 32 are constructed similarly to the control 28, but receivetheir control signals from the triggers 36 and 38, respectively, ratherthan from the trigger 34,

In operation, during one half-cycle of each full cycle of alternatingcurrent input, the anode of the SCR 190 will be positive. If, sometimeduring that position half-cycle, a positive pulse trigger signal isreceived on the lead 174', the SCR 190 will become conductive. Thus,current will flow in a counterclockwise direction around the circuit tocause the motor 20 to rotate in one direction and at a speed determinedby the point in time during the positive half-cycle of input voltagethat a trigger pulse was received on the lead 174'. The SCR 190 willcontinue to conduct during the remainder of that positive half-cycleafter it has received the trigger and will cease conduction only whenits anode become negative with respect to its cathode. As previouslypointed out in connection with the description of FIG. 5, triggersignals cannot be provided simultaneously on both the pairs of leads172, 174 and leads 174, 174. Thus if signals are received on the leads172, 174, the SCR 192 will not become conductive and current will flowin only one direction through the motor 20.

On the other hand, if it is desirable to have the motor rotate in theopposite direction, the control system will provide trigger signals onthe leads 172, 174 and not on the leads 172', 174'. In this case,current will circulate in a clockwise direction through the circuit tocause the motor to rotate in an opposite direction from the firstexample given.

The same operational example applies to the SCR controls 30, 32 whichoperate on the other phases 26S2, 26S3 of the transformer secondary.'Aswas previously pointed out, a positive signal can appear on theconductor 174 only when the anode of the SCR is positive. Similarly, apositive signal can appear on the lead 174 only when the anode of theSCR 192 is positive. Thus, positive turn on the desired SCR is assured.

It is pointed out that a drive circuit would be provided for each slidemotor drive of a multiple-slide machine tool. However, each circuit neednot be provided with its own power input transformer. For example, iftwo 5-horsepower DC motors are'used, a single lO-horsepower powertransformer may be employed with two drive circuits connected inparallel to the transformer.

It is now apparent that the motor drive circuit of the inventionprovides a system that permits the motor to be quickly reversed in itsdirection of rotation or its speed changed without stopping the motor.The system provides self-braking of the motor if the load causes themotor speed to increase. In addition, the system involves no relativelyslow-acting control relays, and is inexpensive to construct. Because ofits solidstate construction, the system to construct. Because of itssolid-state construction, the system is made virtually troublefree.

I claim:

1. In a speed control system for DC motors having rectifiers forregulating the power applied to the motor according to the differencesignal between reference and motor feedback signals, modifying means forlimiting the accelerations and deceleration of said motor, andadjustable dead band control means for rendering the system insensitiveto difference signals less than a preset amplitude, said modifying meanscomprises a capacitor means and switch means responsive to saiddifference signal for charging and discharging said capacitor.

2. The system of claim 1, wherein said switch means comprise transistormeans.

3. The system of claim 2, wherein said transistor means include aPNP-transistor for charging said capacitor means and an NPN-transistorfor discharging said capacitor means.

4. The system of claim 3, wherein said difference signal providesemitter voltage to said transistors, and said capacitor means comprisesa capacitor that is common to base-emitter circuits of both saidtransistors.

5. In a speed control system for DC motors having electronicallycontrolled rcctifiers for automatically regulating the power applied tothe motor according to the difference signal between reference and motorfeedback signals, the improvement comprising electronic modifying meansfor limiting the acceleration and deceleration of said motor, saidmodifying means including an electronic time delay circuit and switchmeans for limiting any change in said difference signal to less than apredetermined rate, said modifying means comprises capacitor means andsaid switch means responsive to said difference signal for charging anddischarging said capacitor.

6. The system of claim 5, wherein said switch means comprise transistormeans.

7. The system of claim 6, wherein said transistor means include aPNP-transistor for charging said capacitor means and

1. In a speed control system for DC motors having rectifiers forregulating the power applied to the motor according to the differencesignal between reference and motor feedback signals, modifying means forlimiting the accelerations and deceleration of said motor, andadjustable dead band control means for rendering the system insensitiveto difference signals less than a preset amplitude, said modifying meanscomprises a capacitor means and switch means responsive to saiddifference signal for charging and discharging said capacitor.
 2. Thesystem of claim 1, wherein said switch means comprise transistor means.3. The system of claim 2, wherein said transistor means include aPNP-transistor for charging said capacitor means and an NPN-transistorfor discharging said capacitor means.
 4. The system of claim 3, whereinsaid difference signal provides emitter voltage to said transistors, andsaid capacitor means comprises a capacitor that is common tobase-emitter circuits of both said transistors.
 5. In a speed controlsystem for DC motors having electronically controlled rectifiers forautomatically regulating the power applied to the motor according to thedifference signal between reference and motor feedback signals, theimprovement comprising electronic modifying means for limiting theacceleration and deceleration of said motor, said modifying meansincluding an electronic time delay circuit and switch means for limitingany change in said difference signal to less than a predetermined rate,said modifying means comprises capacitor means and said switch meansresponsive to said difference signal for charging and discharging saidcapacitor.
 6. The system of claim 5, wherein said switch means comprisetransistor means.
 7. The system of claim 6, wherein said transistormeans include a PNP-transistor for charging said capacitor means and anNPN-transistor for discharging said capacitor means.
 8. The system ofclaim 7, wherein said difference signal provides emitter voltage to saidtransistors, and said capacitor means comprises a capacitor that iscommon to base-emitter circuits of both a said transistors.